According to the preamble of claim 1, this invention relates to a method for adjusting the transmission ratio for a continuously variable transmission of a motor vehicle.
A continuously variable transmission according to the continuously variable principle (CVT transmission) usually consists, among other things, of a starting unit, a forward/reverse drive unit, an intermediate shaft, a differential, hydraulic and electronic control devices and a variator. The variator usually comprises a primary and a secondary pulleys also called primary and secondary sides wherein both pulleys are formed by cones disposed in pairs and are provided with a torque-transmitting continuously variable element which rotates between the two cone pairs. In such a transmission, the actual ratio is defined by the moving radius of the continuously variable element which, in turn, is a function of the axial position of the bevel discs.
Consequently, during a change of ratio, depending on the construction, the cone pairs of primary and secondary pulleys of the variator are alternatively and complementarily separated from each other and joined with each other, via corresponding control elements, which produces a change of the moving radius of the continuously variable element upon the bevel discs and thus a change of the ratio between primary and secondary sides.
In the prior art, the variator was mostly functionally divided between the primary side for control of the ratio change (adjustment) and the secondary side for setting the necessary pressures (pressing), which ensures the required contact between the disc and the continuously variable element so that no slip results.
EP 0 634 590 A1 has disclosed a method and a hydraulic control for ratio adjustment of a continuously variable CVT in which the minimum contact pressure required, which is calculated from the actual ratio and the torque actually to be transmitted to the variator, is covered as needed either via the pressure on the secondary disc or via the pressure on the primary disc. The hydraulic control of the variator is designed as here -series connection of a secondary valve directly coupled with the oil pump and a primary valve, wherein the secondary pressure corresponds to the pump pressure and the primary pressure at no time can fall below the secondary pressure. In the operating states in which the primary disc determines the system pressure, the secondary pressure is correspondingly raised in order to ensure the minimum contact pressure required on the primary disc.
To prevent slip between the disc and the continuously variable element, the prior art teaches to apply great safety additions to the calculated control pressure values. This procedure has the disadvantage of impairing the efficiency of the automatic transmission.
Besides, due to the limitation of the controlled variable for introducing a ratio change on the primary side of the variator, the adjustment dynamics is significantly limited.
Controlled loop structures are usually employed such as described in the Applicant""s DE 196 06 311 A1. Such control loop structures combine a linearization of the control system based on a physical-mathematical model by means of a correction member with a linear Proportional Integral Differential Regulator (PID controller). The controlled variable of the PID controller is interpreted as standard for the adjustment gradient to be set.
On the basis of the prior art, the problem on which this invention is based is to indicate a method for setting the, ratio value for a continuously variable transmission so as to obtain the best possible efficiency of the transmission utilizing at the same time all the degrees of freedom of the adjustment dynamics and simultaneously obtaining the best possible protection of the transmission.
The inventive method must adapt the control pressures of the primary and secondary sides of the variator to an operating point determined so that the control pressures do not fall below calculated minimum contact pressure values required for operation of the variator, thereby reducing to a minimum the effects of an overpressure that are relevant to the efficiency of the variator and protecting the variator from control pressures less than the required contact pressures.
Accordingly, it is proposed that the primary and secondary sides of the variator be regarded, in relation to the adjustment of hydraulic pressure on the variator disks, as being absolutely equal between both disks of the variator and to calculate the operating pressure for both discs, on the basis of the actual ratio and of the actual torque to be transmitted, a minimum contact pressure force pressing on the continuously variable element. It is also proposed to determine, according to the calculated contact pressure minimum force, the primary or, secondary disc as controlled variable to be used. According to the invention, this is determined by comparing the required pressure values with the calculated contact pressure minimum values.
The contact pressure minimum values (theoretical values) are calculated, according to the invention, on the basis of a physical-mathematical model.